Process for the Manufacture of Tooth Parts from Dental Grade Metal Powder

ABSTRACT

When producing tooth replacements from metal, the conventional method of casting is almost exclusively employed, even though said method is a relatively complicated one. The object of the invention is therefore to make available a method that is considerably simpler. In this method, an aqueous suspension, still containing microcrystalline wax and ethoxylated alcohols, is generated with the dental-grade metal powder. The metal powder is deposited on a model by electrophoresis, resulting, for example, in the formation of a cap ( 4 ). This cap ( 4 ) is filled with investment compound ( 5 ) and placed on a firing support ( 6 ), where it is sintered in this position. A cap is obtained which, in terms of its strength characteristics, is the equal of a cap that has been produced by casting. Alternatively, the cap ( 4 ) can be stabilized, during sintering, on a plated stump or in a muffle filled with a temperature-resistant powder.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a submission to enter the national stage under 35U.S.C. 371 for international application number PCT/DE/2006/000134having international filing date 31 Jan. 2006, for which priority wasbased upon patent application 10 2005 025 589.2 having a filing date of3 Jun. 2005 filed in Germany and patent application 10 2005 052 113.4having a filing date of 2 Nov. 2005 filed in Germany.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

(Not Applicable)

REFERENCE TO AN APPENDIX

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a process for the manufacture of tooth partsfrom dental grade metal powder.

2. Description of the Related Art

The lost wax casting technique is still predominantly used for themanufacture of caps for individual teeth, frameworks for bridges, andfull metal crowns, whereby a wax mold, whose shape corresponds to theobject that is to be cast, is modeled on a working model. The hollowzone is formed in a casting mold with the help of this wax model,whereby the casting mold then has metal poured into it. Since thisprocess is very expensive, several attempts have already been made toarrive at the desired shape via the sintering of metal powder.

In accordance with FR 2 660 224 A1, the model of a tooth stump is coatedwith platinum foil while preserving the shape of the stump, and then anadhesive is applied to this foil, whereupon a layer of small nickelchromium globules with diameters of 50-300 μm is applied to theadhesive. A form of sintering then takes place as a result of heating.The sintered cap can then be removed from the platinum foil.

U.S. Pat. No. 5,362,438 A describes a process for mixing metal powderinto, e.g., propylene glycol in order to manufacture a paste that isapplied to a fire resistant model on which it is sintered. Titanium,among others, is indicated to be a material for the metal powder,whereby the titanium is coated with a metal protective layer (palladium)in order to prevent oxidation during sintering.

A further process for the manufacture of tooth parts from metal powderis freeform laser sintering, which is described in EP 1 358 855 A1 andEP 1 021 997 A1.

However, none of the previous processes has been able to gain acceptancerelative to the lost wax process. One reason for this resides in theirsimilar operating expense and increased costs.

Over the last several years, electrophoresis has gained markedly inimportance for the manufacture of full ceramic crowns and bridges. Thefollowing publications are designated as being representatives of thistechnology: WO 99/50 480 A1, DE 100 21 437 A1, DE 101 27 144 A1, DE 10339 603 A1, and WO 2004/04 1113 A1. Since the objective of theseinventions is the manufacture of full ceramic dental prostheses, theinclusion of dental grade metal powder as a material for dentalprostheses was initially ruled out in the case of these known processes.Moreover, it could not be expected that metal powder could be applied bymeans of electrophoresis since short circuits in the electrophoresiscell would probably arise because of the conductivity of the metal.

The problem for the invention, which is indicated in Claim 1, is to makeavailable a simple process for the manufacture of tooth parts fromdental grade metal.

Advantageous embodiments of the invention are described in the dependentClaims 2-13.

The invention will be elucidated in more detail by means of an exampleand FIGS. 1 and 2.

BRIEF SUMMARY OF THE INVENTION

The invention is a process for the manufacture of tooth parts fromdental grade metal powder. It is characterized by the fact that a metalpowder layer is deposited from a suspension liquid onto a model (1) bymeans of electrophoresis, whereby this metal powder layer has the shapeof the desired tooth part (4). The tooth part (4) is stabilized for thesintering process by way of the following steps: it is affixed to afiring support (6); or it remains on a duplicated model; or it is placedin a muffle that has been filled with embedding material ortemperature-resistant powder.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The following aspects are shown:

FIG. 1 shows the electrophoretic coating of a tooth stump model;

FIG. 2 shows a cap that is located on a firing support.

DETAILED DESCRIPTION OF THE INVENTION

A device that is known as such for the electrophoretic coating of toothmodels is shown schematically in FIG. 1. A stump 1 of a working modelor, alternatively, a duplicated model is immersed in a suspension 3 thatcomprises a liquid and a dental grade metal powder, whereby thissuspension is located in a conductive vessel 2. A deposit, whichcorresponds to the shape of a cap, is formed as a result of theapplication of a voltage. In this connection, the steps which aredescribed below are taken in a particular case.

First of all, the model 1 is manufactured from gypsum plaster or anothersuitable material and then thinly coated with a release agent in orderto ensure the easier withdrawal of the as yet non sintered cap. Asuitable release agent is described in DE 198 12 664 A1. The releaseagent is then coated with a conductive layer, e.g., a conventionalsilver lacquer, in order to ensure conductivity for electrophoresis.

In the search for a suitable suspension aid, it has been found that pureor almost pure water is unsuitable, whereby this is in contrast to theelectrophoresis of ceramic powder. Petrolite D 900 (a registeredtrademark of the Baker Hughes firm) has proven to be especially suitablefor the electrophoresis of metal powder. Petrolite D 900 comprises 30 60wt % of microcrystalline wax, 5 10 wt % of ethoxylated alcohols withmore than 14 C atoms in the chain, and 30 60 wt % of water.

A metal powder with the name Nicrobaz 135 (a registered trademark of theWall Colmonoy firm) was used as the metal powder. It is a nickelcompound with 1.9% B, 3.5% Si, and max. 0.06% C. The particle size isaround 2 3 μm.

Electrophoresis at 30-40 V and 20-90 mA took place following themanufacture of a suspension comprising Petrolite and approximately 20 wt% of Nicrobaz, whereby this led within seconds to the formation of a capon the model stump. Owing to the release agent, the as yet extremelyfragile cap can be lifted off the stump. The interior of the cap is thenfilled with the embedding composition 5 as shown in FIG. 2, and theworkpiece is placed on a firing support 6. A fully ready to use cap isformed after sintering at 1125° C. using a sintering time of 8-10 min.The sintering temperature is naturally governed by the material, and itis not permitted to exceed the fusion temperature.

It has been found that stabilization of the tooth part 4 does notnecessarily have to take place on a firing support. If the model 1 is aduplicated model, then the tooth part can naturally remain on theduplicated model during the sintering process, whereby this does howeverlead as a consequence to the destruction of the duplicated model. Theduplicating of the working model is, however, associated with otherdisadvantages such as mold defects and extra work.

Since the majority of metals are prone to oxidation during sintering, itis recommended that sintering be carried out either under vacuum or in aprotective gas, such as argon, or that the ceramic powder or the preshaped framework be coated with a conventional oxidation-stoppinglacquer.

A third possibility for sintering is to put the tooth part into a mufflethat has been filled with embedding material. Because of its higherspecific gravity, the tooth part sinks in the embedding material. Themuffle is then sintered in an oven. The removal of the sintered toothpart from the muffle and the cleaning of the tooth part in terms ofremoving the embedding material do not present any difficulty.

Instead of the embedding material that is known to the dentaltechnician, use can also be made of a temperature-resistant powder. Amixture comprising alumina powder and graphite powder with a 5-50%proportion of graphite has proven to be especially suitable. Thegraphite hereby prevents oxidation of the metal. Compaction of thepowder via the application of pressure, e.g., by means of apressure-operated plunger device, is also recommended prior to thesintering process.

The process in accordance with the invention is usable for allconventional dental grade metals, especially those based on gold. Thespecial advantage of the process in accordance with the invention isthat with appropriate adaptation, it is usable for all theelectrophoresis processes that have actually been developed for themanufacture of full ceramic tooth parts. Mention is made merely of theabove-cited WO 2004/041113 as being representative of these processes.

1. Process for the manufacture of tooth parts from dental grade metalpowder, characterized by the fact that a metal powder layer is depositedfrom a suspension liquid onto a model (1) by means of electrophoresis,whereby this metal powder layer has the shape of the desired tooth part(4), and the tooth part (4) is stabilized for the sintering process byway of the following steps: a) it is affixed to a firing support (6) orb) it remains on a duplicated model or c) it is placed in a muffle thathas been filled with embedding material or temperature-resistant powder.2. Process in accordance with claim 1, characterized by the fact thatthe suspension liquid comprises 30-60 wt % of microcrystalline wax, 5-10wt % of ethoxylated alcohols with more than 14 C atoms, and 30-60 wt %of water.
 3. Process in accordance with claim 2, characterized by thefact that the model is coated with a release layer.
 4. Process inaccordance with claim 3, characterized by the fact that a conductivelayer is applied to the release layer.
 5. Process in accordance withclaim 4, characterized by a particle size of the metal powder of 1-10μm.
 6. Process in accordance with claim 5, characterized by the factthat electrophoresis is carried out at 30-40 V.
 7. Process in accordancewith claim 6, characterized by the fact that electrophoresis is carriedout at 20-90 mA.
 8. Process in accordance with claim 7, characterized bygold as the basic metal for the dental grade metal powder.
 9. Process inaccordance with claim 8, characterized by the fact that the tooth partis a cap (4) or a bridge.
 10. Process in accordance with claim 9,characterized by the fact that sintering is carried out under vacuumand/or the metal powder is coated with an oxidation stopping lacquer.11. Process in accordance with claim 10, characterized by the fact thatsintering takes place in a protective gas.
 12. Process in accordancewith claim 11, characterized by the fact that the temperature-resistantpowder comprises a mixture that comprises a metal oxide and an oxygenconsuming powder.
 13. Process in accordance with claim 12, characterizedby the fact that use is made of a mixture that comprises alumina powderand graphite powder.